1. Field of the Invention
This invention relates to a scanning optical apparatus and an image forming apparatus using the same, and chiefly to an image forming apparatus such as a laser beam printer or a digital copier having the electrophotographic process adapted to deflect a beam emitted from light source means by a deflecting element, and light-scan a scanned surface through an imaging element having an fθ characteristic to thereby record image information, and a scanning optical apparatus suitable for use in such apparatus.
2. Related Background Art
In the scanning optical apparatus of a laser beam printer (LBP) or the like, a beam emitted from light source means in conformity with an image signal is light-modulated. This light-modulated beam is periodically deflected by a light deflector comprising, for example, a polygon mirror, and is converged into a spot shape on the surface of a photosensitive recording medium by an imaging optical system having an fθ characteristic while its distortion is corrected, and that surface is light-scanned to thereby effect image recording.
FIG. 6 of the accompanying drawings is a schematic view of a scanning optical apparatus according to the prior art. In FIG. 6, a divergent beam emitted from light source means 1 to which an image signal modulated by modulating means has been inputted is made into a substantially parallel light by a collimator lens 2, and enters a cylinder lens 4 with the cross-sectional shape of the beam limited by an aperture stop 3. That portion of the parallel beam having entered the cylinder lens 4 which is in the main scanning surface emerges in its state as it is. Also, in the sub-scanning surface, the beam converges and is formed substantially as a linear image on the reflecting surface of a light deflector 5 comprising a polygon mirror. The beam reflected and deflected by the reflecting surface of the light deflector 5 is directed to a scanned surface 8 through two scanning optical elements (fθ lenses) 6 having an fθ characteristic. The light deflector 5 is then rotated in the direction of arrow, whereby the scanned surface 8 is scanned in the main scanning direction and is scanned in the sub-scanning direction by the next reflecting surface of the light deflector 5. A part of the beam deflected by the light deflector 5 is reflected by a reflecting mirror 75 through the scanning optical elements 6 and is directed to a slit 71 and a sensor 72, and is synchronized with the writing beginning timing of an image.
In recent years, with the higher resolution and lower cost of image forming apparatuses having the electrophotographic process, it has become the mainstream to manufacture the scanning optical element (fθ lens) of the aforedescribed scanning optical apparatus by plastic molding, and use an inexpensive fθ lens in which chromatic aberration of magnification is not compensated for. Also, in order to cope with high-speed color image formation, there is required a scanning optical apparatus for use in a tandem type color image forming apparatus as shown in FIG. 7 of the accompanying drawings wherein a plurality of scanning optical apparatuses 11–14 are used at a time correspondingly to four colors, e.g. Y, M, C and K to record image information of each color on each of different photo-sensitive drums 21–24.
However, in the scanning optical apparatus for forming a final image by the beams from the plurality of scanning optical apparatuses, a variation attributable to the chromatic aberration of magnification of the fθ lenses is caused in the scanning magnification among the plurality of scanning optical apparatuses by                (a) the initial wavelength deviation among the laser light sources of the respective scanning optical apparatuses,        (b) the wavelength deviation by the mode hopping of semiconductor laser resulting from a change in the environment, and        (c) the fluctuation of the refractive index of a plastic lens by the change in the environment, and this leads to the deterioration of images. FIG. 6 of the accompanying drawings shows the image area when the wavelength of the light source of a plurality of scanning optical apparatuses has changed, and the deviation of the imaged position of a beam at a write beginning detecting position. In FIG. 6, there is shown an example in which as indicated at image formation points A and A′, the deviations of the image formation points of respective lasers occur on a scanned surface 8.        
Such a deviation of the imaged position on the scanned surface during the variation in the scanning magnification causes a small pixel deviation on the left end side (the lower side in FIG. 6) of the image near the detecting position because in the actual image, as shown in FIG. 6, synchronism is taken at the writing beginning position detecting position, and conversely becomes a great pixel deviation (scanning magnification deviation) on the right end side (the upper side in FIG. 6) of the image. Consequently, a pixel deviation is caused in the whole of the image printing area, and yet the magnification of the image changes.
This also holds true in a scanning optical system in the tandem type color image forming apparatus, and when a change in magnification occurs among the plurality of scanning optical apparatuses, the image formation point of cyan C and the image formation point of black B coincide with each other on the left end side of the image, as shown in FIG. 8, but the registration deviation (color deviation) among respective colors becomes great on the right end side of the image, and this causes the deterioration of the image. While in FIG. 8, color deviations in B (black) and C (cyan) are shown, the same also applies in the case of the color deviation between the other colors Y (yellow) and M (magenta).